Linear Pulsejet Combustion simulation.

[hinote]

In this example it's not uncommon during combustion to have the velocity zero crossing point further down the exhaust pipe with a region of gas flow higher than the gas combustion velocity separating the two. With a poorly designed Type02 combustor it's possible to generate 2 possible locations for combustion, unburnt fuel being propelled towards the velocity zero crossing point to burn again at this second location.
Graham.

Graham:

Thanks for observing this effect. I've encountered exactly the same thing when using UFlow to design my motors; the graphic visualization is especially useful in separating the "false zero crossovers" from the returning wave.

I've chosen to stay with the wave crossover as my point of determination--and this has served me well in my designs.

[Graham C. Williams]

Finally found the time to make some major changes.

The major problem had been the detection and movement of the limits of combustion.
The realisation that there are two types of combustion has helped in that I can quickly test the routine with both types of combustor.
A simple calculation shows by how much it is reasonable for the limits of combustion to move.
For example, a jet 1.0m long modelled by 255 cells:
Each cell is about 3.92e-3m long.
Lets say we’re looking at the flame front. The gas velocity at the point of interest is 12m/s and the calculation Timestep is 0.000007s.
In one Timestep the point moving at the local gas velocity will move 8.4e-5m. This movement is less than one cell.
The first conclusion: It’ll take a number of Timesteps each with a slightly different gas velocity for the flame front to move one cell.
Combustion time: A 50mm dia. combustion chamber will burn in about 0.0013sec. At this Timestep combustion will take about 186 iterations.

The combustion location cannot just track the movement of the ideal combustion conditions when those conditions are moving faster than the charge can be allowed to move.
I can only come to the conclusion that the conditions and location set in the very early part of combustion are largely held throughout the combustion time. But, if the conditions change so the gas velocity is higher than the allowed flame speed, combustion must be effectively switched off while the charge flows to a new location suitable for combustion. We have seen this effect.

My detection routines are very good at tracking the location of the ideal combustion conditions at all stages during combustion. I needed to impose a realistic flame front movement in the direction of the movement of the combustion conditions. Partial constant volume combustion can take place when two things happen:
The gas velocity is below the flame speed. The gasflow from the exhaust and induction pipes is flowing in and oppose combustion.
Heat release, temperature rise but no pressure rise can take place where:
The gas velocity is below the flame speed. Gas is flowing away from this point.

More later.
Graham.

[Graham C. Williams]

something like this:

:
+FL = Upper combustion flame speed
-FL = Lower combustion flame speed
Induction is on the Left
Exhaust on the Right
The arrows within the upper and lower flamespeed limits show the gasflow direction.
The vertical lines indicate the limits of combustion along the pipe.
A, B and C are the 3 Common types.
A' is related to A.

Graham.